One of the most debilitating aspects of neurodegenerative diseases in humans is the loss of neural elements that mediate cognitive abilities. Previous work has established the importance of cholinergic projections from the basal forebrain to the hippocampus in the maintenance of cognitive function, and the loss of cholinergic cells in the septal region that contribute to the septohippocampal pathway is one of the hallmarks of Alzheimer's disease. Despite considerable progress in characterizing pathological changes that occur in the septohippocampal pathway during aging, the underlying cellular mechanisms of cholinergic neurodegeneration remain unclear. An alternative approach is to identify cellular factors that regulate the survival and maturation of the septohippocampal pathway during development, which may provide clues about molecular mechanisms required to support the survival of these neurons later in life. In order to gain a complete understanding of the mechanisms underlying this process it will be necessary to simplify the variables and partition the highly complex set of regulatory events that exist in the whole animal into experimentally approachable entities. The goal of this project is to develop and validate an in vitro organotypic primate model system for studying neural plasticity, and to initiate experiments that will clarify the cellular and molecular events underlying hormonal control of neural development in the primate septohippocampal pathway. Both steroid and thyroid hormones influence cognitive abilities and appear to exert profound effects on the structure and neurochemistry of the septohippocampal pathway. Moreover, key components of this system differ between primates and other mammalian animal models. Thus, it is likely that unique mechanisms participate in regulating neuronal development of the septohippocampal system in humans and nonhuman primates. However, developmental studies are rarely attempted in primates because they require the use of large numbers of animals to ensure reliable results, and many of the necessary molecular reagents are not available. The proposed project will utilize the unique resources of the ORPRC to overcome these limitations by isolating new molecular probes and developing an in vitro model system that will allow multiple experimental manipulations to be carried out on tissue samples derived from a single rhesus macaque, and which provides in vitro conditions that accurately reflect developmental events as they occur in vivo. First, with the assistance of the ORPRC molecular services, we will isolate cDNAs that correspond to each of the major steroid and thyroid hormone receptors expressed in rhesus macaque brain. In cooperation with the surgical staff of the Division of Primate Medicine, we will collect hippocampal tissue from fetal rhesus macaques and use morphometrics, immunohistochemistry, in situ hybridization, and RT-PCR to determine whether hippocampal neurons in fetal explants, cultured under defined conditions, display the morphological features and patterns of hormone receptor gene expression that characterize hippocampal neurons developing in vivo (Specific Aim 1). Second, we will utilize this new model system to begin studying possible interactions between gonadal and thyroid hormones on the development of cholinergic afferents to the hippocampus from the basal forebrain. For this, explants of the basal forebrain will be co-cultured with hippocampal explants under varying hormonal conditions, and the development of cholinergic projections from the basal forebrain to the hippocampal formation assessed by using the fluorescent tracer Dil in combination with immunohistochemistry for choline acetyltransferase (Specific Aim 2). This project will, therefore, establish the validity of a powerful model system that can be used to study neuronal development in primates. Future studies will utilize this experimental model to examine the hypothesis that sex steroid and thyroid hormones influence neuronal survival and maturation in the hippocampus by regulating the expression of hormone and receptor subtypes and growth factors during key developmental critical periods. The results of this line of research will make a significant contribution to our emerging understanding of the role of hormones in regulating neural development, and may provide clues about unique cellular mechanisms in primate species underlying cognitive deficits that reflect neonatal abnormalities, or that accompany neurodegenerative disease.